Interstitial fibrosis of the heart contributes to cardiac dysfunction and morbidity associated with heart failure. The specific cellular origins and molecular pathogenesis of cardiac fibrosis are not well defined, but the end state of fibrosis includes increased expression of extracellular matrix (ECM) proteins by interstitial fibroblasts. During heart development, the interstitial fibroblasts arise from epicardial-derived cells (EPDCs) that invade the myocardium, differentiate, and synthesize ECM ofthe fibrous cardiac matrix. There is recent evidence that EPDCs are activated and recapitulate developmental mechanisms in adult cardiac injury and pathologic fibrosis. Wnt/p-catenin signaling is required for epicardial development, but its role in fibroblast cell lineage development or cardiac fibrosis has not been examined. We hypothesize that Wnt/p-catenin signaling promotes normal development of interstitial fibroblasts and also contributes to pathologic interstitial fibrosis in adult cardiovascular disease. Using constructs and disease models developed by Projects 1 and 3, as well as in our own laboratory, the three Specific Aims are designed to test the necessity and sufficiency of the roles played by Wnt/p-catenin fibroblast signaling during development and cardiac disease. Specific Aim 1 will determine if Wnt/p-catenin signaling promotes interstitial fibroblast lineage differentiation and ECM formation during development in vivo. Aim 2 will determine if Wnt/p-catenin signaling promotes maladaptive fibrotic disease and if Wnt pathway inhibition is sufficient to prevent the progression of fibrosis and heart failure in adulthood. Aim 3 will examine the intersection of Wnt/p-catenin and TGFp/Smad2/3 signaling pathways in the induction and progression of cardiac fibrosis. These in vivo mechanistic studies will be used to dissect the molecular contributions of Wnt/p-catenin signaling to cardiac fibroblast lineage, development and the pathogenesis of interstitial fibrosis in cardiovascular disease. The long-term goal of the proposed studies is to identify new therapeutic targets for the treatment and management of maladaptive cardiac fibrosis and subsequent heart failure. 1 RELEVANCE (See instructions): Cardiac fibroblasts are a major cell type in the heart but the roles they play in terms of disease development are largely unexplored. We hypothesize that specific signaling processes that are critical for normal development play an important role in defining these cells. We also want to test if these processes are more generally involved irji cardiac disease and heart failure. Definitive proof of these ideas will identify new therapeutic possibilities for the treatment of cardiac disease.